Field of the Invention
The present invention relates to an SOHC-type valve operating system in an internal combustion engine having a single cam shaft commonly provided for a pair of intake valves and a pair of exhaust valves, a plurality of intake valve driving members rockably disposed between the cam shaft and the pair of intake valves and having an operative-connection switchover mechanism for switching over the connection and disconnection, and a pair of exhaust valve driving members rockably disposed between the cam shaft and the pair of exhaust valves and independently corresponding to each of the exhaust valves.
Description of the Prior Art
Conventionally such a valve operating system has been known as disclosed, for example, from Japanese Patent Application Laid-open No. 1405/92.
Such a conventional valve operating system includes three valve side rocker arms, two of which are operatively connected, at their one ends, to a pair of intake valves. Rollers in rolling contact with a cam are rotatably supported on the other ends of the rocker arms, so as to reduce frictional loss. However, since an operative-connection switchover mechanism having a rocker arm is provided between one end of the rocker arms of each intake valve and the rocker arms of the pair of exhaust valves to be operatively connected individually to the pair of exhaust valves are arranged at both sides of the three rocker arms of the side of the intake valves, an ignition plug must be disposed between the rocker arms on the side of both the exhaust valves. However, if the ignition plug is disposed at the exhaust side, it is difficult to improve thermal extraction of the ignition plug. As a result, in order to prevent the occurrence of knocking, the ignition timing cannot be set near an ignition timing for generating a maximum torque, and there arises a problem in which a sufficient torque is not obtained. Further, when the above-described structure is applied to a vehicle-mounted V-type internal combustion engine in which an intake side is disposed inwardly, there also arises a problem in which the space for attaching and detaching the spark plug is inadequate around the internal combustion engine.